This invention relates to nondestructive testing of bodies by magnetic particle inspection procedure, for detection of defects and discontinuities in the surface of the body, and is particularly concerned with a method for achieving high sensitivity magnetic inspection of bodies by employing relatively inexpensive non-fluorescent magnetic particles, and to magnetic particle compositions which can be employed in such method.
Conventional magnetic particle inspection methods for detection of defects, flaws and discontinuities in objects or parts, generally utilize two type of magnetic particles for this purpose, namely, non-fluorescent-type particles usually employed in low sensitivity inspection methods, and fluorescent-type magnetic particles employed in high sensitivity inspection methods for detection of minute flaws, defects or microcracks. Also, conventional magnetic particle inspection methods generally do not employ a contrasting background against which the oriented magnetic particles indicating flaw locations are observed, but generally rely on the contrast between the oriented magnetic particles and the color of the part itself.
In my U.S. Pat. Nos. 3,243,876 and 3,344,345 there is disclosed a magnetic inspection method which employes a continuous intermittent alternating current which is not transmitted through the part, in a printing process, and wherein the periodic reversal in the direction of the magnetic force results in violently impinging the magnetic particles against the part surface, leaving a printed mark at the points of impingement, following which the magnetic particles are removed from the surface and inspection is made of the printed pattern resulting from such procedure. According to the patents, the workpiece can be initially coated with an opaque material prior to application of the magnetic particles on the object. However, the printing procedure of these patents is particularly designed to obtain detailed metallurgical information, for example detailed information as to the honeycomb structure below a facing sheet or below the surface of the body, and requires specialized equipment for application of a suitable intermittent alternating current to provide the magnetic force to cause violent impingement of the magnetic particles against the object surface to produce the printing smears. In many applications of magnetic inspection particle procedures where detailed metallurgical information such as that noted above is not required, and where only surface flaws or discontinuities are required to be detected, it is desirable to employ less sophisticated and relatively inexpensive conventional magnetic inspection equipment and procedure.
Accordingly, a chief object of the present invention is the provision of a magnetic inspection procedure employing relatively inexpensive non-fluorescent magnetic particles together with conventional magnetic inspection equipment, for detecting surface cracks, flaws and discontinuities, with high sensitivity, particularly where minute cracks or microcracks are present.